Where are the source files used to create a Boot CD ?

I need bootable media to default to Acronis True Image, not Windows after the 15 second timeout. I can edit the "bootmenu.xml" menu on a bootable Flash Drive but I want to do the same on a Boot CD.

So where are the source files used for creating the Boot Media. The Acronis folder in C:\Program Files (x86)\Acronis does not have anything that looks like the Boot CD files and this folder is very small compared to the CD content. And I don't see any type of Archive file in the Acronis folder that looks like it would have the Boot media.

I have read that ImgBurn can be used to edit an ISO file but I can't see that feature.